My hard drive crashed but I was able to use a USB case to copy the files to a new computer. However now some (10%?) of flac files aren't showing up in foobar. When I look in the folder they are there, with flac file names, but they won't play.

Is there anything I can do to make these files playable?

Edit: tested on my J3 and while it shows up, it doesn't play and says "invalid file."

If the hard drive failed, it's likely that the data is simply corrupted and you're out of luck.

There are data recovery tools out there, but I've never used them. And I suppose the recoverability of errors depends on what the specifics are - sometimes its just the file system that gets corrupted, sometimes the disk head actually crashes into the platter and physically scrapes the surface of the disk.